The present invention relates to a system for the metered feeding of volatile fuel components, in particular into the intake manifold of an internal combustion engine of a motor vehicle. The system includes an electromagnetic canister purge valve which is actuatable in proportional pulse-width modulation, and a closed-loop control circuit with which, for the purpose of compensating for external interferences, the coil current of the canister-purge valve is adjustable.
To collect the volatile fuel vapors contained in the fuel tank of a motor vehicle, to store them, and to finally feed them in a metered manner into the intake manifold of the internal combustion engine of the motor vehicle is known. As a rule, an activated carbon canister is used for collecting and storing the fuel vapors. Activated carbon binds (absorbs) the fuel vapor and, at appropriate ventilation, releases it again. Thus, fresh air, sucked through the activated carbon canister by the engine during driving operation, takes up the fuel and feeds it to the engine. Metered addition of the air/fuel mixture takes place via a canister-purge valve. A canister-purge valve is typically an electromagnetic valve including a solenoid, an armature having a sealing element, a sealing seat, as well as a restoring spring (closing spring). In the energized state, using the magnetic force of the coil, the solenoid lifts the sealing element from the sealing seat against the spring resistance of the closing spring, thereby unblocking the flow-through opening. The valve is in its open position. In the de-energized state, the sealing element is pressed onto the sealing seat by the closing spring and the valve is in its closed position.
Actuation of the canister-purge valve takes place via the engine control unit which, corresponding to the current load state of the internal combustion engine, determines the maximum meterable air/fuel mass flow and converts it into a corresponding control signal. As a rule, the canister-purge valve is actuated in a clocked manner (pulse-width modulation) and, as a function of the predetermined pulse-duty factor specified in each case by the engine control unit, releases different metered quantities. Pulse-duty factor is understood in this connection as the ratio between the period of the open valve and the total period, i.e., the period of the open and closed valve.
In order to avoid the oscillating movement of the armature including the sealing element between the open and closed position, which involves various disadvantages, it is also known, from WO 99/06893, for example, the entire disclosure of which is incorporated by reference herein, to operate the clocked canister-purge valve in what is known as proportional operation. Here, the clock frequency is to be selected at such a high level that the valve can no longer follow the oscillating excitation and remains instead in a position which corresponds to the average coil current.
Due to manufacturing tolerances, aging, fluctuations in the vehicle electrical system, and temperature, e.g., through self-heating of the coil, purely controlled canister-purge valves have a relatively large tolerance range in their characteristic curve (mass flow as a function of the actuation time). This results in the fact that, in order to be able to reliably prevent too high a mass flow through the canister-purge valve, the possible purge performance cannot be fully utilized. On the other hand, tightening of the exhaust gas emission regulation is imminent, which makes an increase in the purge performance absolutely necessary. However, an increase in the purge performance is very hard to achieve and involves a highly complex application, i.e., very complex treatment algorithms are devised in order to obtain maximum purge performance.
In order to compensate for interfering influences on the valve's metering accuracy caused by fluctuations in the vehicle electrical system or temperature fluctuations, WO 99/06893 A1 describes a very complex closed-loop control circuit in which the instantaneous coil current is determined, digitized, and supplied to a microprocessor situated in the engine control unit. In a complicated algorithm composed of this signal and additional input variables, this microprocessor ultimately determines the control signal for the coil current.